1. Field of the Invention
The present invention relates to a fuel injection system for use in a diesel engine and, more particularly, to a fuel injection system having provisions for controlling the rate of injection of the system in accordance with varying operating condition of the engine.
2. Description of the Related Art
It is well known in the art that, during a light load, low speed operating condition such as idling of a diesel engine, the level of combustion noise and the amount of nitrogen oxide emissions may be effectively reduced by performing, for each fuel injection cycle, a small quantity fuel injection prior to a main injection pulse. This small quantity fuel injection pulse preceding the main injection pulse is commonly referred to as a pilot injection.
In the copending U.S. Pat. Application Ser. No. 754,750, filed July 15, 1985 and assigned to the assignee of the present invention, there is disclosed a fuel injection system having an arrangement adapted to control the rate of injection of the system in such a manner that a pilot injection is effected when the engine is operating under a light load low speed condition. This arrangement includes a variable volume chamber defined by a movable piston which is translated by an electronically controlled piezoelectric actuator having a stack of piezoelectric elements. The variable volume chamber is in permanent fluid communication with a pumping chamber of a fuel injection pump so that any pressure rise in the pumping chamber due to the delivery stroke of a pump plunger is reflected in the variable volume chamber. Fuel injection takes place in the following manner. As the delivery stroke of the pump plunger proceeds to increase the fuel pressure in the pumping chamber, the pressure in the variable volume chamber is concurrently increased, causing the piston to squeeze the piezoelectric stack of the actuator, thereby developing an electric potential across each piezoelectric element. When the fuel pressure in the pumping chamber reaches a predetermined value, a fuel injection nozzle opens to start fuel injection. Immediately thereafter, the piezoelectric actuator is controlled to discharge the accumulated electric potential, thereby allowing the piezoelectric stack to contract and permitting the piston to retract under the action of the fuel pressure in the variable volume chamber. This brings about a slight expansion of the volume of the variable volume chamber, causing the fuel pressure in the pumping chamber to be lowered below the opening pressure of the injection nozzle, whereby fuel injection ceases for a short period, thereby forming a pilot injection. As the delivery stroke of the pump plunger proceeds further, the fuel pressure in the pumping chamber is re-established so that a main injection takes places shortly after the pilot injection pulse. Control of the piezoelectric actuator may be performed in various other ways.
Although this arrangement is very effective in controlling the rate of injection, several problems have been encountered:
(1) Due to the small Young's modulus of the piezoelectric elements, the piezoelectric stack of the actuator undergoes unavoidable contraction during the main injection period. This limits the fuel pressure delivered by the injection pump and causes a reduction in the injection quantity and an extension in the injection period. These problems are particularly serious under a heavy load, high speed operating condition wherein the fuel pressure rises to about twice the pressure developed during the idling condition.
(2) Since the piezoelectric elements are repeatedly subjected to a high load due to the high fuel pressure in the variable volume chamber, there is a tendency for the elements to be degraded due to a loss of polarization.
(3) As the piezoelectric elements are compressed under an extremely high load and develop a high potential, it is necessary to design a drive circuit for driving the piezoelectric actuator in such a manner that is will sufficiently withstand a high voltage.
(4) The piezoelectric elements undergo a breakdown of voltage because a high voltage is developed across each element having a small thickness.